1. Field of the Invention
This invention generally relates to telecommunications signals. More particularly, this invention relates to apparatus and methods for limiting jitter in a first telecommunications signal which can be caused by pointer movement in a second telecommunications signal into which the first telecommunications signal is being mapped. The invention finds specific application in the limiting of jitter in a TUG-3 signal caused by a pointer movement in a SONET STM-1 (STS-3) signal, although it is not limited thereto.
2. State of the Art
The telecommunications network servicing the U.S. and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwdith requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper cable and with lower attenuation.
While fiber optic cable represents the future in telecommunications, presently there remains an entire telecommunications network comprised of various cable types, served by equipment of different vintages, and run according to various coexisting transmission standards. While older standards, cables, and equipment will be eventually phased out, for the time being, it is necessary that all the old and new standards, equipment, and transmission lines be as compatible as possible. Often, such compatibility requires the mapping and multiplexing of a plurality of lower speed signals into higher speed signals and vice versa. Such a mapping with concurrent multiplexing and demultiplexing is common in commonly found add/drop equipment.
When mapping one signal into another signal, it is desirable to limit signal jitter, both because it is advantageous to limit the amount of buffer memory and resultant delay which results from large buffers, and because the standards of various telecommunications signals do not permit large amounts of jitter. In telecommunications signals subject to the SONET standards, jitter can be caused by pointer movements (e.g., pointer increments (INC) and decrements (DEC)) which cause byte stuffing and destuffing. Stuffing (INC) and destuffing (DEC) are legitimate mechanisms which compensate for frequency offset between two signals. When stuffing or destuffing occurs, the SONET payload bytes move relative to the overhead bytes. In order to accommodate stuffing (INC) or destuffing (DEC) in the SONET signal, the telecommunications signals which are being mapped into the SONET signal must either provide additional data (in the case of destuffing in the SONET signal) than would otherwise be provided, or hold data back (in the case of stuffing in the SONET signal) which would otherwise be provided. The provision of additional data, or the holding back of data, however, must be assimilated by the signal being mapped into the SONET signal, e.g., through bit stuffing or destuffing. This assimilation, however, can cause substantial jitter in the signal being mapped, and cause it to fail jitter performance requirements.